1. Field of the Invention
The present invention relates generally to stimulating medical devices, and more particularly, to an implantable electrode assembly for a stimulating medical device.
2. Related Art
Hearing loss is generally of two types, namely conductive and sensorineural. The treatment of both of types of hearing loss has been quite different, relying on different principles to deliver sound signals to be perceived by the brain as sound. Conductive hearing loss occurs when the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. In such cases, hearing loss is often improved with the use of conventional hearing aids, which amplify the sound so that acoustic information reaches the cochlear hair cells. Such hearing aids utilize acoustic mechanical stimulation, whereby the sound is amplified according to a number of varying techniques, and delivered to the inner ear as mechanical energy. This may be through a column of air to the eardrum, or through direct delivery to the ossicles of the middle ear.
On the other hand, sensorineural hearing loss is due to the absence or destruction of the cochlear hair cells which are needed to transduce acoustic signals into auditory nerve impulses. Individuals suffering from this type of hearing loss are unable to derive any benefit from conventional hearing aid systems regardless of the volume of the acoustic stimulus. This is because the natural mechanisms for transducing sound energy into auditory nerve impulses are either absent or damaged. In such cases, cochlear™ implants (also referred to as cochlear™ devices, cochlear™ prostheses, cochlear™ implant systems, and the like; simply “cochlear implants” herein) have been developed to provide the sensation of hearing to such individuals. In cochlear implants, electrical stimulation is provided via stimulating electrodes positioned as close as possible to the nerve endings of the auditory nerve, essentially bypassing the hair cells in a normally functioning cochlea. The application of a stimulation pattern to the nerve endings causes impulses to be sent to the brain via the auditory nerve, resulting in the brain perceiving the impulses as sound.
The electrode assembly is inserted during an operation that usually takes between 2-3 hours depending on the device to be implanted. An incision is made behind the ear to expose the temporal bone; the temporal bone consists of the squamous, the mastoid, the tympanic, zygomatic and petrous segment. Typically, cochlear implants require the opening of the mastoid bone which leads to the middle ear. A shallow recess is then created to hold the implant package in place substantially level with the bone. Next a hole is drilled which allows the surgeon access into the cochlea. This hole, referred to as a cochleostomy, extends from the middle ear to the perilymphatic canals of the cochlea. A cochleostomy is typically formed through the round window, the oval window, the promontory or through the apical turn of the cochlea. The electrode assembly is then gently threaded into the shell-like structure of the cochlea and the incision closed; the cochleostomy remains open and heals with scar tissue over the next few days.
In the carrier member, separately insulated, biocompatible conductive wires (e.g., platinum or platinum/iridium) normally extend from the implanted stimulator unit to the electrodes positioned within the cochlea. Signals received by the implantable components are applied by the electrodes leading to stimulation of the auditory nerve. When implanted, the carrier member is potentially exposed to regular mechanical regular movement over time, such as chewing and talking, eventually leading to failure.